Although methane is abundant, its relative inertness has limited its utility in conversion processes for producing higher-value hydrocarbons. For example, oxidative coupling methods generally involve highly exothermic and potentially hazardous methane combustion reactions, frequently require expensive oxygen generation facilities and produce large quantities of environmentally sensitive carbon oxides. In addition, non-oxidative methane aromatization is equilibrium-limited, and temperatures ≥ about 800° C. are needed for methane conversion greater than a few percent.
In the oxidative coupling of methane (“OCM”), methane and oxygen react at high temperatures over a catalyst to generate ethane as the primary product and ethylene as a secondary product; in the process, the methane feed and the products are partially oxidized to carbon monoxide and carbon dioxide. There are several drawbacks to the OCM reaction, such as low yields of ethane and ethylene (generally not more than about 25%) and a high amount of unreacted methane. Further, the OCM reaction is exothermic and has to be carefully operated to avoid potential runaways/explosions.
The OCM reaction is believed to start with the oxidative scission of one of the C—H bonds in methane leading to the formation of methyl radicals on the surface, followed by desorption and coupling in the gas phase to form ethane; ethane is in turn dehydrogenated by oxygen on the catalyst surface (and depending on temperature, non-oxidatively in the gas phase) forming ethylene. The inherent propensity of the C2 products to oxidize limits the yields.
There have been two major technical hurdles to OCM commercialization: (i) low catalyst activity and selectivity and (ii) lack of catalyst stability at high temperature and steam partial pressure. Low catalyst activity requires higher operation temperatures (800-900° C.) and O2/CH4 ratios, resulting in lower C2 selectivity, higher exotherms and a more hazardous operation. Low catalyst selectivity translates into low carbon efficiencies and uneconomical recovery of ethane/ethylene from highly diluted streams. Lack of catalyst stability is a major barrier to the development of a practical process.
Another reaction in which methane is reacted to produce higher-value hydrocarbons is methane co-aromatization with higher alkanes and alkenes. In this reaction, methane is co-reacted with higher alkanes (typically ethane or propane), ethylene, propylene or alcohols to form a mixture of benzene, toluene and xylenes (“BTX”). As compared to methane aromatization, which is equilibrium-5 limited and only yields high conversions above 780° C., methane co-aromatization occurs at lower temperatures (500-700° C.) and achieves significantly higher methane conversion. For instance, when reacted alone, only 10-20% methane is converted to aromatics (80% selectivity, mostly benzene) at 785° C. By comparison, a 58/42 methane/ethane mixture can be transformed to BTX at 550° C. and 10-30% methane conversion; similarly, a 75/15 methane/propane mixture can be converted to BTX at 600° C. and 12-42% methane conversion. Higher methane conversions have been reported for the co-aromatization of methane/ethylene and methane/propylene mixtures and for the co-aromatization of methane and alcohols. The addition of the co-feed helps activate methane to the point that it is converted to a greater extent at much lower (up to 250° C. lower) temperatures. An example of methane co-aromatization is U.S. Pat. No. 5,936,135, which discloses reacting methane at a temperature in the range of 300° C. to 600° C. with (i) a C2-10 olefin and/or (ii) a C2-10 paraffin in the presence of a bifunctional pentasil zeolite catalyst, having strong dehydrogenation and acid sites, to produce aromatics. The preferred mole ratio of olefin and/or higher paraffin to methane and/or ethane in the feed ranges from about 0.05 to about 2.0.
OCM and methane co-aromatization may be integrated to produce higher yields of desired products, such as aromatics. In U.S. Pat. No. 5,336,825, a two-step process for the conversion of methane to liquid hydrocarbons in the gasoline range is disclosed. In the first step, methane and oxygen are converted to ethylene and lower olefins via OCM in one reactor and the product stream of the OCM reaction is catalytically converted to aromatics and other hydrocarbons in the gasoline range in a separate reactor. A process in which the OCM catalyst and aromatization catalyst were stacked within a single reactor was disclosed in Li et al. (Li et al., “Combined single-pass conversion of methane via oxidative coupling and dehydroaromatization,” Catalysis Letters, vol. 89, pp. 275-279, 2003).
Of the aromatics, para-xylene is of particular value since it is useful in the manufacture of teraphthalic acid, a major component of polyester fibers and resins. Thus, commercially, once the aromatic mixture containing benzene, toluene, xylenes and heavier aromatics is produced, further processing is required to maximize para-xylene yields. Examples of such processes are benzene and/or toluene alkylation, toluene disproportionation and xylene isomerization. The catalysts used in at least some of these processes may be selectivated to increase the yield of para-xylene over other xylenes.
There is interest in developing alternative routes for the conversion of methane into aromatics and particularly routes that allow more methane to be incorporated into the aromatic product in a more efficient manner than the prior art processes. A process that controls the potential hazards of oxidative coupling of methane and utilizes the high amount of unconverted methane would be advantageous.